Motor 1919?

[Paultrehberg]

Is this a 1919 motor?

[MMasa]

Yes. It's a magneto ignition and is also a model that left the factory with the newly designed clutch installed on the transmission.
The lack of an L before the 19 indicates the magneto part (the presence of an L would mean that it had a generator and battery for Lighting)
The "A" between the 19 and the serial number indicates the new style clutch.
This was a mid-year change to the clutch precipitated by a patent infringement lawsuit filed by Eclipse.
Out of curiosity, does your engine also have USA stamped into it in a few places? If yes, then it's a US military bike.
Mark Masa

[Ken41WLA]

I think I see an A in the top left of the photo, at the top of the aluminum case. As I recall, the HD museum has a 1919 FUS, with USA stamped below the front cylinder.